Etching of silicon nitride

ABSTRACT

Silicon nitride is etched employing a composition containing a fluoride containing compound, certain organic solvents, and water.

TECHNICAL FIELD

The present invention is concerned with etching silicon nitride andparticularly etching silicon nitride at a rate at least as fast as thatfor silicon dioxide. In addition, the present invention is concernedwith certain etching compositions that are suitable for etching siliconnitride at a rate at least as fast as that for silicon dioxide.

BACKGROUND ART

In the fabrication of microelectronic components, a number of the stepsinvolved, for instance, in preparing integrated circuit chips and thepackaging for the chips (articles to which the chips are attached andprotected), are etching processes. Accordingly, over the years, a numberof vastly different types of etching processes to remove material,sometimes in selective areas, have been developed and are utilized tovarying degrees. Moreover, the steps of etching different layers whichconstitute, for instance, the finished integrated circuit chip are amongthe most critical and crucial steps.

One method widely employed for etching is to overlay the surface to beetched with a suitable mask and then immerse the surface and mask in achemical solution which attacks the surface to be etched, while leavingthe mask intact and while only etching other materials of the article toat most, a minimum extent.

In many structures, a silicon nitride layer exists on top or beneath asilicon dioxide layer and it becomes necessary to etch the siliconnitride without overetching the silicon dioxide. The ability to use afluoride such as hydrogen fluoride in such situation presents a problemsince hydrogen fluoride etches silicon dioxide at a much faster ratethan it does silicon nitride. It would therefore be desirable to providean etching procedure that achieves excellent removal of the siliconnitride and especially a procedure that exhibits an etching rate forsilicon nitride that is at least as fast as the rate for silicondioxide.

SUMMARY OF INVENTION

The present invention is concerned with a method for etching siliconnitride. The process of the present invention comprises contacting anarticle that contains silicon nitride and silicon dioxide with anetching composition that contains about 0.1 to about 3 molar of afluoride containing compound, an organic solvent and about 0.1 to about4 molar of water to thereby etch the silicon nitride at a rate at leastas fast as the rate for the silicon dioxide.

In addition, the present invention is concerned with certain preferredetching compositions that are capable of etching silicon nitride at arate at least as fast as the rate for silicon oxides. The preferredcompositions of the present invention contain about 0.1 to about 3 molarof a fluoride containing compound, an organic solvent, and about 0.1 toabout 4 molar of water.

The organic solvents employed in the present invention typically haverelatively high flash point and provide low viscosity compositions.Suitable solvents include sulfolanes, oxolanes, esters, ketones,aldehydes, lactones, halogenated hydrocarbons, monohydric alcohols,amines and imides.

Other objects and advantages of the present invention will becomereadily apparent to those skilled in this art from the followingdetailed description, wherein it is shown and described only thepreferred embodiments of the invention, simply by way of illustration ofthe best mode contemplated of carrying out the invention. As will berealized, the invention is capable of other and different embodiments,and its several details are capable of modifications in various obviousrespects, without departing from the invention. Accordingly, thedrawings and description are to be regarded as illustrative in natureand not as restrictive.

BEST AND VARIOUS MODES FOR CARRYING OUT INVENTION

The present invention is concerned with etching a silicon nitride.Moreover, the silicon nitride is present in an article that alsoincludes silicon dioxide, wherein both the silicon nitride and silicondioxide will come in contact with the etching composition. The presentinvention provides for etching the silicon nitride at a rate at least asfast as the rate for silicon dioxide and preferably at a rate greaterthan that for silicon dioxide and most preferably at a rate at leastabout 10 percent greater.

The silicon dioxide is preferably high density silicon dioxide. Highdensity silicon dioxide refers to thermally grown silicon dioxide orsilicon dioxide that is chemically vapor deposited (CVD) or physicallyvapor deposited (PVD) followed by thermal annealing.

The etching compositions employed pursuant to the present inventioncontain a compound containing fluoride and an organic solvent. Theamount of the fluoride containing compound in the composition is about0.1 to about 3 molar, and preferably about 0.15 to about 1.5 molar, andmost preferably about 0.2 to about 1 molar.

Typical compounds providing a fluoride source according to the presentinvention are hydrofluoric acid, ammonium fluoride, fluoroborates,fluoroboric acid, tin bifluoride, antimony fluoride, tetrabutylammoniumtetrafluoroborate, and aluminum hexafluoride. Also, a fluoride salt ofan aliphatic primary, secondary or tertiary amine can be used. Such havethe following formula:

    R.sub.1 N(R.sub.3)R.sub.2

wherein R₁, R₂ and R₃ each individually represent H or an alkyl group.

Typically, the total number of carbon atoms in the R₁, R₂ and R₃ groupsis 12 carbon atoms or less. The preferred fluoride compounds arehydrogen fluoride and ammonium fluoride.

The hydrogen fluoride is typically employed as a 49 weight percentaqueous solution.

Examples of suitable organic compounds include oxolanes, sulfolanes,esters, ketones, aldehydes, lactones, halogenated solvents, amines,imides and monohydric alcohols. Examples of suitable esters are estersof carbonic acids, benzoic acid, phthalic acid, isophthalic acid andterephthalic acid, and especially the C₁ -C₆ alkyl esters. Preferredorganic solvents are propylene carbonate, N-methyl pyrrolidone, gammabutyrolactone, methylene chloride, benzyl alcohol, N-formyl morpholine,N-formyl piperidine, cyclohexanone, cyclopentanone, methyl benzoate,diglyme, 2-methyl tetrahydrofuran, and methyl and ethyl esters ofphthalic, isophthalic or terephthalic acids. The more preferred solventsemployed pursuant to the present invention are propylene carbonate,N-methylpyrrolidone and gamma butyrolactone, with propylene carbonatebeing the most preferred.

Organic solvents employed in the present invention provide for lowviscosity compositions, which in turn render the compositions moreworkable and providing for better etch uniformity. This is especially sowhen compared to using polyhydric alcohols such as glycerol and ethyleneglycol.

The compositions according to the present invention also include water.Typically, the water is employed in an amount of about 0.1 to about 4molar, preferably about 0.15 to about 2 molar, and most preferably about0.2 to about 1.5 molar.

The etching process of the present invention is typically carried out attemperatures of about 20° C. to about 90° C., and preferably about 30°C. to about 70° C. The temperature employed is directly dependent uponthe fluoride concentration. In particular, as the fluoride concentrationincreases, it becomes necessary to also increase the temperature inorder to achieve the desired etching rate of the silicon nitride incomparison to that for the silicon dioxide. For instance, at a fluorideconcentration of 0.25 molar, to achieve an etch rate for the siliconnitride that is as fast as that for the silicon dioxide, the temperatureis about 55° C.; whereas, at a concentration of 1.5 molar, thetemperature is about 70° C. The minimum temperature to use for anyspecific fluoride concentration can be determined without undueexperimentation by those skilled in the art once aware of thisdisclosure.

The ability to use fluoride to etch silicon nitride at a rate equal toor greater than that for silicon dioxide is surprising in view of theprior experience with hydrogen fluoride in etching silicon dioxide at amuch higher rate than that for silicon nitride.

The following non-limiting examples are presented to further illustratethe present invention.

EXAMPLE 1

An etching composition is prepared by admixing one part by volume of a49 percent by weight aqueous solution of HF with about 112 parts byvolume of propylene carbonate to provide a 0.25 molar HF solution inpropylene carbonate. A blanket silicon wafer having a layer thermallygrown silicon dioxide beneath a layer of silicon nitride is contactedwith the etching composition. The article contains deep trenches therebyexposing the walls of the underlying silicon dioxide layer to theetchant. The etching is carried out at a temperature of about 60° C. forabout 20 minutes. The etching solution etches the silicon nitride at arate of about 0.42 nanometers/minute and the silicon dioxide at a rateof about 0.36 nanometers/minute, resulting in a selectivity of siliconnitride etching relative to the silicon dioxide of about 1.2.

EXAMPLE 2

An etching composition is prepared by admixing one part by volume of a49 percent by weight aqueous solution of HF with about 70 parts byvolume of propylene carbonate to provide a 0.4 molar HF solution inpropylene carbonate. A blanket silicon wafer having a layer thermallygrown silicon dioxide beneath a layer of silicon nitride is contactedwith the etching composition. The article contains deep trenches therebyexposing the walls of the underlying silicon dioxide layer to theetchant. The etching is carried out at a temperature of about 65° C. forabout 11 minutes. The etching solution etches the silicon nitride at arate of about 1.2 nanometers/minute and the silicon dioxide at a rate ofabout 1.0 nanometers/minute, resulting in a selectivity of siliconnitride etching relative to the silicon dioxide of about 1.2.

In this disclosure, there are shown and described only the preferredembodiments of the invention but, as aforementioned, it is to beunderstood that the invention is capable of use in various othercombinations and environments and is capable of changes or modificationswithin the scope of the inventive concept as expressed herein.

What is claimed is:
 1. A method for etching silicon nitride whichcomprises contacting an article containing said silicon nitride and asilicon dioxide with an etching composition containing about 0.1 toabout 3 molar of a fluoride containing compound, an organic solvent,wherein said solvent is selected from the group consisting of propylenecarbonate, N-methyl pyrrolidone, gamma butyrolactone, methylenechloride, benzyl alcohol, N-formyl morpholine, N-formyl piperidine,cyclohexanone, cyclopentanone, methyl benzoate, diglyme, 2-methyltetrahydrofuran, methyl and ethyl esters of acid selected from the groupconsisting of phthalic acid, isophthalic acid and terephthalic acid andabout 0.1 to about 4 molar water to thereby etch said silicon nitride ata rate at least as fast as that for said silicon dioxide.
 2. The methodof claim 1 wherein said organic solvent is selected from the groupconsisting of propylene carbonate, N-methyl pyrrolidone and gammabutyrolactone.
 3. The method of claim 1 wherein said solvent ispropylene carbonate.
 4. The method of claim 1 wherein said fluoridecontaining compound is selected from the group consisting ofhydrofluoric acid, ammonium fluoride, fluoroborates, tetrabutylammoniumtetrafluoroborate, fluoroboric acid, aluminum hexafluoride, tinbiflouride, antimony fluoride and fluoride salt of an aliphatic primary,secondary or tertiary amine.
 5. The method of claim 1 wherein saidfluoride containing compound is hydrofluoric acid or ammonium fluoride.6. The method of claim 1 wherein said fluoride containing compound ishydrofluoric acid.
 7. The method of claim 1 wherein said silicon dioxideis high density silicon dioxide.
 8. The method of claim 7 wherein saidsilicon-dioxide is selected from the group consisting of thermally grownsilicon dioxide, chemically vapor deposited thermally annealed silicondioxide and physically vapor deposited thermally annealed silicondioxide.
 9. The method of claim 7 wherein said silicon dioxide isthermally grown silicon dioxide.
 10. The method of claim 1 wherein theamount of water is about 0.15 to about 2 molar.
 11. The method of claim1 wherein the amount of water is about 0.2 to about 1.5 molar.
 12. Themethod of claim 1 wherein the amount of said fluoride is about 0.15 toabout 1.5 molar.
 13. The method of claim 1 wherein the amount of saidfluoride is about 0.2 to about 1 molar.
 14. The method of claim 1wherein the contacting with said composition is carried out at atemperature of about 20° C. to about 90° C.
 15. The method of claim 1wherein the contacting with said composition is carried out at atemperature of about 30° C. to about 70° C.
 16. The method of claim 1wherein the concentration of said fluoride and the temperature employedare such that the etch rate for said silicon nitride is greater than theetch rate for said silicon dioxide.
 17. The method of claim 16 whereinthe etch rate for said silicon nitride is at least about ten percentgreater than for said silicon dioxide.